1. Field of the Invention
The present invention relates to a synchronization method between radio sets in a wireless communication network system provided with mobile radio sets mounted on a mobile body and a plurality of fixed radio sets installed along a moving route of the mobile body to sequentially relay and transmit information, where these radio sets perform communication in a time division multiple access method, and particularly relates to a synchronization method for a wireless communication network system, that is capable of maintaining a synchronized state of each radio set even when a communication failure occurs between fixed radio sets arranged to be communicable with each other.
2. Description of Related Art
In a wireless communication network system provided with a base station for transmitting information into a control area and a plurality of radio stations wirelessly connected to this base station, it is necessary for the respective stations to be synchronized with each other in order to transmit and receive information in the time division multiple access method. As the synchronization method, it is considered a method in which all the radio stations receive a synchronization packet transmitted from the base station at the same time to achieve synchronization, or a method in which all stations within the control area receive a synchronization signal (e.g., GPS signal) at the same time to achieve synchronization. However, the method using the GPS signal requires expensive GPS receivers to be installed at all stations, which not only increase costs but also are unusable within tunnels. Furthermore, the method using the synchronization packet cannot be used in a wireless communication network system for a wide control area where radio waves from the base station cannot reach some stations such as a control area having a one-dimensional range like a case of railway train control.
Therefore, as a synchronization method in which there is no need to provide receivers for the synchronization signal at all stations and which can be applied even to a wide control area where radio waves from the base station cannot reach some radio stations, there is a synchronization method for a wireless communication network as disclosed in Japanese Laid-Open Patent Application Publication No. 2000-102063. The synchronization method disclosed in Japanese Laid-Open Patent Application Publication No. 2000-102063 is such that each radio station sequentially relays and transmits a synchronization packet transmitted from a base station to synchronize all stations including the radio stations where radio waves from the base station do not reach.
In the meantime, in the synchronization method disclosed in Japanese Laid-Open Patent Application Publication No. 2000-102063, adjacent ground radio sets perform wireless communication sequentially to relay the synchronization packet. Therefore, for example, when a train enters a tunnel to narrow the propagation space of radio waves in the tunnel due to the vehicle body and hence to attenuate the radio waves, or when each antenna of ground radio sets is located at a position lower than the vehicle height of the train so that the propagation of radio waves between ground radio sets located in front of and behind the train will be blocked by the vehicle body and this will make it hard to receive the radio waves, the relay operation of the synchronization packet cannot be performed and this may result in being incapable of maintaining the synchronized state between radio sets.